Fix alert dialogs in VpnSettings.

Fix alert dialogs in VpnSettings.

+ Don't check mDialogFragment.isVisible() in SettingsPreferenceFragment.removeDialog()
  as mDialogFragment may not be visible in parent fragment's onResume().
+ Replace mConnectDialog with mConnectDialogShowing and remove
  removeConnectDialog().
+ Dismiss alert dialogs in onPause() so that we don't need to maintain extra
  states during pause-resume cycle.

In addition, fix a NPE when startVpnTypeSelection().

Bug: 3381434
Bug: 3289365

Change-Id: Ic4aa87c7a618d95e86e45d6617f2ad7dab35f019